The author is deeply grateful to Raman Sanyal from whom he learned most of the material. These notes combines the original material straight from [1]} together with Sanyal lectures in the MSRI Summer School Positivity Questions in Geometric Combinatorics in summer 2017.
The author also thanks Takayuki Hibi and Akiyoshi Tsuchiya for the kind hospitality and the organization of the Summer Workshop on Lattice Polytopes 2018.
